This application claims priority from Japanese Serial No. 279131/99, filed Sep. 30, 1999.
This invention relates to a thermosetting adhesive composition and a structural adhesive which uses the same.
In packaging integrated circuits (IC), thermosetting adhesive compositions are often used for bonding the electronic parts. These thermosetting adhesive compositions can be a reactive hot-melt adhesive composition or just a reactive hot-melt composition. Such adhesive compositions form bonds that have superior heat resistance due to a crosslinking reaction (curing) which is initiated by heating or irradiation after applying pressure.
As shown in Japan Patent No. H 9-137028 or No. H 10-316925, when the reactive hot-melt adhesive includes polyethylene or another polyolefin, conductivity is reduced, outgassing increases, and humidity-absorption is worse. At the same time, it controls mixing of impure ions.
In detail, Japan Patent No. H 9-137028 describes a reactive hot-melt composition which includes: (a) a thermo plastic polymer which includes polyolefin which has epoxy groups; and (b) a copolymer obtained from irradiation-polymerization of a monomer that consists of acryl compounds which include aliphatic alkyl (meth)acrylate and functional groups that reacted with thee epoxy groups. This reactive hot-melt composition has features such as: (1) it controls the generation of reaction byproducts; (2) it does not require solvent when it is molded into film; (3) it can be stored at room temperature with high stability; and also (4) it does not need to be exposed to radiation when pressure is applied and can be used for parts where radiation can not reach.
The relative amount of this monomer component which forms copolymer (b) in this reactive hot-melt composition is high compared to the whole composition. Typically, the monomer component comprises 15 to 60 weight percent of the precursor, which includes this monomer component and a thermoplastic polymer. It is hard to mix with the thermoplastic polymer. Accordingly, the monomer component has a tendency to bleed out before the precursor is exposed to radiation. In addition, part of the monomer component above is volatile and causes out gassing and bad odor. It is especially not preferred for bonding electronic parts. Therefore, in such case, the precursor is generally cured using high levels of radiation in order to avoid out gassing and bad odor of the unreacted monomer component due to bleed out.
Japan Patent No. H 10-316955 describes a thermo-setting adhesive which includes: (a) ethylene-glycidyl (meth)acrylate copolymer; (b) ethylene-alkyl (meth)acrylate copolymer; and (c) rosin which has a carboxyl group. This thermosetting adhesive differs from the reactive hot-melt composition in Japan Patent No. H 9-137028, and it does not include a volatile and reactive monomer component. Therefore, high levels of radiation are not thought to be necessary in order to avoid bad odor and out gassing.
However, in order to obtain the desired fluidity for hot pressing the hot-melt and thermo-setting adhesive above, it is necessary to use high levels of radiation (for example, electron beam). For example, Japan Patent No. H 10-316955 describes a relatively powerful electron beam (150 kGy) which is used to irradiate the thermosetting adhesive. In this case, improved crosslinking of the polyethylene is expected, but the main or side chains of each component are partially cut, and there is a tendency to generate volatile products or ions with low boiling points which can cause bad odor and out gassing. In addition, when the thermosetting adhesive is used with films set on a liner, there is tendency to foam due to the high levels of radiation which are transformed into heat energy. Foaming could occur during hot pressing or post curing. In any case, it not only causes bad appearance of the film, but it also may reduce the bond strength. Furthermore, using electron beam at high power levels with thermosetting adhesives is not generally effective.
In order to reduce the power level of the electron beam required, addition of electron beam sensitizers has been well known. Typical electron beam sensitizers for use with polyethylene are triallyl cyanurate (TAC), triallyl isocyanurate (TAIC), or trimethylol propane trimethacrylate (TMPTMA). All of these electron beam sensitizers usually have vinyl groups or (meth)acrylate groups that react easily in response to electron beam. They contribute to improved crosslinking in the thermosetting adhesive. As a result, it is expected that electron beam power can be reduced and still obtain the required fluidity.
However, in the case when this electron beam sensitizer is added to thermosetting adhesive composition listed in, for example, Japan Patent No. H 10-316955 (compared to the case where electron beam sensitizer is not added), there are cases when the amount cannot be reduced in order to obtain the desired fluidity. Even if it could be reduced, bond strength is lowered in accordance with it, and the adhesive properties could be degraded.
Therefore, the object of this invention is to offer a thermosetting adhesive composition with the desired features which is useful for bonding electronic parts that may be cured by a low-power electron beam.
In one aspect, the invention provides a thermosetting adhesive composition comprising:
(a) ethylene-glycidyl (meth)acrylate copolymer;
(b) rosin with a carboxyl group; and
(c) (meth)acrylate with a carboxyl group.
In another aspect, the invention is an article or structure comprising a first adherent, a second adherent, and a layer of the thermosetting adhesive of invention between the first adherent and the second adherent bonding the first and second adherents together. Preferably, these layers are bonded by hot pressing or laminating.